Classicly, Mississippi River multi-mode containers move in barges connected in strings which are moved up and down river by towboats. Negotiating locks requires separating the strings, making a "double locking" raising and lowering the water twice, then refastening the barges back together, a task that requires 1.5 to 2 hours, which adds to transit costs and to the inventory-like costs of goods in transit. Negotiating of locks and "waiting turn" delays account, on the upper Mississippi River, for a substantial part of the total transit time.